This invention relates to dynamic force testing machines, and more particularly to a method for simulating a dynamic force response corresponding to an experimental event, such as a peel test for an adhesive-backed substrate or refastenable system (e.g., a hook-and-loop fastener system) and a method of calibration.
Dynamic force testing machines, such as tensile testing machines (i.e., constant speed of extension machines), commonly perform experiments to measure particular characteristics of materials or objects. These measured characteristics may then be used for further evaluation of the materials or objects. For example, materials may be dynamically tested on tensile testing machines to ascertain their mechanical properties. Such tests are typically performed with multiple samples of different materials, creating a library of measured test data comparing different materials to one another. For such libraries of data to be useful, consistent performance by the testing machine is essential. This is particularly true for dynamic testing machines, where multiple measurements (e.g., force, extension displacement) are recorded over time, generating a dynamic response of a particular characteristic, such as force.
One such experiment is a peel test for a hook-and-loop fastener. When the mating components of a hook-and-loop fastener are peeled apart, the force required to disengage the hooks from the loops varies over time. As graphically depicted, this force typically has a sawtooth or serrated profile that varies over time, caused by gradual increases in the peeling force as individual hooks are plastically deformed, followed by momentary drops in force as the hooks release from respective loops. Consistently reproducing such a sawtooth dynamic force response, or any such dynamic force response, is the focus of the present invention.
Conventional testing machines (e.g., tensile testing machines) performing dynamic testing have suffered from various drawbacks, most notably the inability to calibrate the testing machines to ensure consistent dynamic testing. For instance, performing multiple tests on similar portions of material may yield variability between tests. However, determining whether such variability stems from the testing machine or the material itself, is difficult if not impossible. To minimize variability in the testing machines, those skilled in the art utilize calibration methods. As used herein, ‘calibration’ denotes verification of a machine's accuracy, usually with an accompanying adjustment of the machine to minimize its error. Typical calibration techniques are static. ‘Static calibration’ denotes calibration of a machine where the test specimen or moving elements of the machine are either fixed or change position slowly, such that dynamic effects upon the machine are negligible. Because dynamic effects are not included in the calibration, static calibration techniques cannot accurately calibrate the dynamic response of a particular machine. A machine calibrated statically, yet performing dynamic tests, may or may not be performing accurately. As such, dynamic calibration techniques may be used to better confirm the dynamic performance of a machine. ‘Dynamic calibration’ denotes calibration of a machine where the test specimen or moving elements of the machine change position quickly, such that dynamic effects upon the machine are no longer negligible. Dynamic calibration is useful when applied to a single measured characteristic, or channel (e.g., force, displacement, time), of the testing machine by itself. Beyond dynamic calibration of a single channel by itself, however, dynamic calibration may be more effectively applied to multiple channels simultaneously. Such a multi-channel calibration not only dynamically calibrates the individual channels, it dynamically calibrates their interaction with one another. Without such simultaneous calibration of such channels, individual calibration of each channel separately cannot account for potential changes occurring only when such characteristics are measured simultaneously.
Specifically, conventional static calibration techniques used in conjunction with tensile testing machines involve only static calibration of the force sensing portions of the tensile testing machines, including load cells and any associated recording circuitry of the machines. By moving elements of the tensile testing machine (e.g., extending a crossbar) slowly during the static calibration, the actual dynamic movement of the tensile testing machine, as compared to the desired dynamic movement, is not calibrated. Actual movements of the tensile testing machine must accurately match the desired movements of the machine, however, because movements of the machine are often incorporated into other measured characteristics. For example, tensile testing machines may be used to create a force versus extension curve. Because static calibration only calibrates the ability of the tensile testing machine to measure a single characteristic, or channel (e.g., force, displacement), in a static condition, the measurements reported by the tensile testing machine when both characteristics are measured simultaneously may be inaccurate, casting doubt over the accuracy of the curve. Limiting the calibration to only static or dynamic calibration of individual characteristics by themselves does not sufficiently calibrate the machine for a dynamic test. For instance, many material properties are strain and strain-rate dependent, making extension displacement an important characteristic that should be calibrated simultaneously with force to ensure accuracy. Various ASTM standards specify accuracy requirements for tensile testing machine measurements. The widely accepted ASTM E4 calibration procedure, for example, employs deadweights or highly accurate load cells to calibrate only the force measurement and recording system of the tensile testing machine. Because no other portion of the tensile testing machine is calibrated, however, this process yields only a static calibration of a single characteristic and cannot gauge the true dynamic response of the tensile testing machine. Furthermore, many conventional tensile testing machine software programs have user selectable or configurable sampling rates and data filters for dynamic testing. Mere deadweight calibration of such machines does not ensure that the machine is operating properly for a given dynamic test. In other words, applying conventional calibration methods to dynamic tensile testing machines calibrates only particular individual characteristics of the machine separately from one another, whereas simultaneous dynamic calibration occurs while the machine performs dynamically, thereby calibrating all parts of the machine and measured characteristics together (e.g., load cell and extension together). Applying such a calibration verifies the interaction of individual measured characteristics with one another.
There is a need, therefore, for an apparatus and method capable of accurately dynamically calibrating the various measurement channels of a tensile testing machine, or any testing machine, simultaneously by performing repeatable dynamic testing simulating actual experimental events, such as (but not limited to) the aforementioned peel tests. For instance, such an apparatus and method would dynamically calibrate two or more measurements simultaneously during a simulated dynamic test to verify the accuracy of such measurements when measured together simultaneously. For additional detail regarding apparatus for simulating a dynamic force response, reference may be made to the utility application filed simultaneously by Peter D. Honer, Oliver P. Renier and Peter S. Lortscher, entitled APPARATUS FOR SIMULATING A DYNAMIC FORCE RESPONSE, assigned to Kimberly-Clark Worldwide, Inc., the entire disclosure of which is incorporated by reference in a manner consistent herewith.